When a vehicle is stopped at a stoplight or anywhere else and the engine is running and the car is shaking it's usually an indicator of worn or broken motor mounts or transmission mounts.
Motor mounts and transmission mounts that are broken should be fixed as soon as possible because it could leave you stranded should the motor mounts or transmission mounts break completely.
The motor mounts and transmission mounts have rubber between the metal to help prevent vibrations and shaking and when those motor mounts or transmission mounts are worn out or broke the car will shake when idling.